1. Field of the Invention
The present invention relates generally to a fire sensor, and more particularly to a fire sensor that detects temperature changes in a hot airflow generated by a fire, using a temperature detecting element.
2. Description of the Related Art
A prior art fire sensor, for detecting temperature changes in a hot airflow generated by a fire, is shown in FIG. 56 by way of example (Japanese Utility Model Laid-Open Publication No. SHO55-150490). This fire sensor includes a sensor main body 51 with a circuit board 55 incorporated therein, a protective case 52 made of metal and protruding from the sensor main body 51, and a temperature detecting element 53 housed in the protective case 52. In addition to these components, the fire sensor further includes a heat collecting plate 54 mounted on the tip end of the protective case 52 for purposes of accelerating the speed of a temperature response to a hot airflow generated by a fire. The temperature detecting element 53 consists of a transistor.
FIG. 57 shows another fire sensor that detects temperature changes in a hot airflow generated by a fire. This fire sensor includes a sensor main body 51 having a circuit board 55 incorporated therein, and a temperature detecting element 53. The temperature detecting element 53 consists of a thermistor coated with resin. The fire sensor further includes a protective structure 57 to protect the temperature detecting element 53. In this case, since the temperature detecting element 53 is exposed to air through the resin coating formed thereon, sufficient response speed is obtained without a special structure such as the heat collecting plate 54 shown in FIG. 56.
The above-described fire sensors, however, have the following problem. The fire sensor in FIG. 56 is constructed such that heat does not escape to the sensor main body 51 via the wall of the protective case 52. Because of this, the temperature detecting element 53 has to be disposed away from the sensor main body 51, and consequently, the size of the fire sensor cannot be reduced. In the case of the fire sensor shown in FIG. 57, the temperature detecting element 53 must be disposed away from the sensor main body 51 to prevent thermal energy from escaping via wiring 58. In addition, the protective structure 57 is required because the wiring 58 is low in mechanical strength. Thus, it is fairly difficult to achieve a reduction in sensor size.
Furthermore, there is a prior art fire heat sensor which performs differential heat sensing. This differential fire heat sensor detects a fire by judging the rate of a rise in temperature caused by the fire, using a plurality of temperature detecting elements and a heat conduction structure thereof. As such a differential fire heat sensor, there are a thermocouple type heat sensor and a heat sensor which employs two thermistors. In addition, there is a temperature sensor employing a micro machining technique for purposes of detecting a rapid change in temperature. These differential fire heat sensors employ two temperature detecting elements, and detect the temperature difference therebetween to judge a rapid rise in temperature. To cause the temperature difference to occur, one of the two detecting elements has a high response to heat and the other has a low response to heat.
Such differential fire heat sensors, however, have the following problems.
FIG. 58 shows a thermocouple type heat sensor (Japanese Patent Publication No. SHO 44-24057). In the figure, a semiconductor thermocouple 71 which is a heat sensing element is in contact with a hot junction 73 on the inside of a heat sensing cover 72 made of metal, and is installed in the central portion of the heat sensor. The hot junction 73 and a cold junction 74 are in a positional relationship perpendicular to each other with respect to a sensor mounting surface 75. As the hot junction 73 and the cold junction 74 are in a positional relationship perpendicular to the direction of a hot airflow, sensitivity does not vary depending on the hot airflow direction.
On the other hand, the heat sensing cover 72 is made of metal. Because metal is typically great in thermal diffusivity, the escape of thermal energy through heat transfer is great and a rise in the temperature of the hot junction 73 is small. Since the temperature rise of the hot junction 73 is small, the temperature difference between the hot junction 73 and the cold junction 74 becomes small and only a small output can be obtained.
FIG. 59 shows a prior art heat sensor with two thermistors as heat sensing elements (Japanese Utility Model Publication No. HEI 1-297795). In this type of heat sensor, the magnitude of a temperature difference signal that is obtained from two thermistors 83a, 84a is sufficient because one (thermistor 83a) of the two is exposed to a hot airflow. However, since the two thermistors 83a, 83b are in a positional relationship that is asymmetrical in a horizontal direction, there is a problem that sensitivity (magnitude of the temperature difference) will greatly depend on the direction of a hot airflow.
FIG. 60 shows a temperature sensor employing a micro machining technique for purposes of detecting a rapid temperature change (Japanese Patent Publication No. HEI 7-43284). In the figure, this temperature sensor includes a substrate 91, an insulating layer 91a formed on the top surface of the substrate 91, and sensing elements S and S′ formed on the thick portion A and thin portion A′ of the substrate 91 through the insulating layer 91a. The bottom surface of the substrate 91 is mounted on a heat sink 92. The thickness of the substrate 91 is 400 to 600 μm or less and the insulating layer 91a is 10 μm or less. Since they are on the order of a micrometer, a reduction in sensor size is possible. However, because the sensing elements S and S′ are disposed in close proximity to each other, there is a problem that the temperature difference therebetween is small. If the sensing element S is disposed away from the sensing element S′ to obtain a great temperature difference, sensitivity (magnitude of the temperature difference) will depend on the direction of a hot airflow and the sensor will be increased in size and cost.